This invention relates to a transmission device which transmits power from a driving side to a driven side.
Conventionally, a transmission device for transmitting a rotation from a power at the same rotational speed, at an increased rotational speed or at a decreased rotational speed has been used mainly as a speed change device. As such a speed change device, the following examples are enumerated.
FIG. 10 (a) is a schematic representation showing a general gear type multistage speed change device, wherein the gear type multistage speed change device 61 has an input shaft 63 connected to a motor 62 as a driving side and a driven shaft 64 as a driven side. Said input shaft 63 is provided with a first speed input shaft gear 65 and a second speed input shaft gear 66 and said driven shaft 64 is provided with a first speed driven shaft gear 67 engaging with said first speed input shaft gear 65 and a second speed driven shaft gear 68. Furthermore, the gear ratio is preset such that a gear ratio of the first speed input shaft gear 65 to the first speed driven shaft gear 67 is different from a gear ratio of the second speed input shaft gear 66 to the second speed driven shaft gear 68. In addition, when the second speed input shaft gear 66 and the second speed driven shaft gear 68 come into an engagement by a displacement of said driven shaft 64 (FIG. 10 (b)), a rotational speed transmitted from the input shaft 63 to the driven shaft 64 will be changed stepwise.
As an example of belt type, a belt type stepless speed change device 71 (as shown in FIG. 11) is used. Said belt type stepless speed change device 71 has an input shaft 73 connected to a motor 72 as a driving side and a driven shaft 74 parallel to the input shaft 73 as a driven side. Said input shaft 73 and said driven shaft 74 are provided with an approximately conical pulley 75, 76, respectively, wherein a belt 77 transmitting a power from the input shaft 73 to the driven shaft 74 is wound around both pulleys 75, 76. Therefore, the rotational speed of the driven shaft 74 can be changed steplessly when the difference of peripheral speed between both pulleys 75 and 76 is changed by a displacement of the winding location of said belt 77 in the axial direction of the inputs shaft 73 and the driven shaft 74.
Furthermore, as an example of a stepless speed change device, a friction disc type stepless speed change device 81 (as shown in FIG. 12) is used. Said friction disc type stepless speed change device 81 is provided with an input shaft 83 connected to a motor 82 as a driving side and a driven shaft 84, perpendicular to the input shaft 83, as a driven side, wherein the input shaft 83 is provided with an input shaft disc 85 witn a larger diameter and the driven shaft 84 is provided with a driven shaft disc 86 abutted against the input shaft disc 85. In said friction disc type stepless speed change device 81, the rotational speed of said driven shaft 84 can be changed steplessly by using the difference of the peripheral speed between the input shaft disc 85 and the driven shaft disc 86 by a displacement of said driven shaft disc 86 in the radial direction of said input shaft disc 85 in the same way as said belt type stepless speed chabge device 71.
However, in such a prior transmission device, e.g. in the case of said gear type multistage speed change device 61 (FIG. 10), the rotational speed of said driven shaft 64 cannot be changed steplessly because power is transmitted by a respective combination of gears having a different gear ratio. Since in order to obtain a plurality of rotational speeds from said driven shaft 64 a plurality of gears is required, the capacity and the weight of the device are, therefore, inadvantageously increased. Furthermore, since in a speed change operation it is required to interrupt a transmission of power and to obtain a correct matching between the crest of one gear and the root of the other gear, the speed change operation is intricate and the device itself must be complicated.
In the case of said belt type stepless speed change device 71, it is possible to change the rotational speed of said driven shaft 74. However, since in the case of the belt type stepless speed change device 71 a power is transmitted by a friction force generated in a contact surface between said belt 77 and both pulleys 75 and 76, the power transmission efficiency becomes lower, compared with said gear type multistage speed change device 61, due to a partial consumption of transmission power by said friction force. If a large torque is required from said driven shaft 74, a plurality of said belts 77 shall be provided, thereby a volume ratio of the device to the required torque is inadvantageously increased. A deterioration of the material used in the contact surface portion between said belt 77 and both pulleys 75, 76 is a problem and, in particular, when a load torque of the driven shaft 44 is large, a great degree of deterioration is expected so that it is difficult to use it for a device requiring a large torque at the driven shaft 74. Furthermore, since in the case of said friction disc type stepless speed change device 81 a power is also transmitted by a friction force, the power transmission efficiency is low in the same way as said belt type stepless speed changer device 71, so that it is difficult to use it for a device requiring a large torque at the driven shaft 84.